Nice question. Here are a few reasons and explanations.

1. As mentioned, it would only allow for take-offs in one direction, and if you put slopes on both ends, it would use up a lot of runway, because landing aircraft would have to overfly the vertical cliff of the ramp on the approach ends. Very dangerous.

2. One of the purposes of these ramps on aircraft carriers is to prevent launching the aircraft directly into the water. Aircraft carriers pitch up and down especially in rough seas. When an aircraft is being launched it takes a couple seconds and it may not be possible to pefectly time it so that the bow (front) of the ship is pointing up to the sky. If the seas are rough (as anyone with sailing experience can attest) there are moments where the bow is pointing at the surface of the ocean, event if its for just a few seconds, if that is the point when the aircraft is launched it could impact the water. So a slope helps in that scenario. Larger aircraft carriers that are more stable, and have a higher deck above water level can get away without a slope.

3. Aircraft carriers are not just runways, they are catapults! So, aircraft are pushed up the slope and launched into the air. Putting a slope on a runway, means that the aircraft's engine has to use up even more power climbing the uphill slope, which may likely slow it down even more, and the increased pitched means its pitching up and possibly slowing down which is a recipe for a stall. What a small aircraft on a very short runway needs is more speed not more pitch. Because we don't have catapults a ramp is like taking off up hill, which will eat up some of our airspeed when the aircraft leaves the ramp. If the aircraft has a surplus of speed such that it would be above stall speed by the time it finished climbing the ramp, then it would, work, but in that case, the aircraft had surplus speed, and could have just taken off normally.

4. Cost. It would be very expensive to add a ramp to a small airport.

Hope this helps.

Nice question. Here are a few reasons and explanations.

1. As mentioned, it would only allow for take-offs in one direction, and if you put slopes on both ends, it would use up a lot of runway, because landing aircraft would have to overfly the vertical cliff of the ramp on the approach ends. Very dangerous.

2. One of the purposes of these ramps on aircraft carriers is to prevent launching the aircraft directly into the water. Aircraft carriers pitch up and down especially in rough seas. When an aircraft is being launched it takes a couple seconds and it may not be possible to pefectly time it so that the bow (front) of the ship is pointing up to the sky. If the seas are rough (as anyone with sailing experience can attest) there are moments where the bow is pointing at the surface of the ocean, even if its for just a few seconds, if that is the point when the aircraft is launched it could impact the water. So a slope helps in that scenario. Larger aircraft carriers that are more stable, and have a higher deck above water level can get away without a slope.

3. Aircraft carriers are not just floating runways, they are catapults! So, aircraft are pushed up the slope and launched into the air. Putting a slope on a runway, means that the aircraft's engine has to use up even more power climbing the uphill slope, which may likely slow it down even more, and the increased pitched means its pitching up and possibly slowing down which is a recipe for a stall. What a small aircraft on a very short runway needs is more speed not more pitch. Because we don't have catapults, a ramp is like taking off up hill, which will eat up some of our airspeed when the aircraft climbs the ramp. If the aircraft has a surplus of speed such that it would be above stall speed by the time it finished climbing the ramp, then it would work, but in that case, the aircraft had surplus speed, and could have just taken off normally before getting to the ramp.

Hope this helps.